SAR, Cell Phones, and San Francisco

An interesting article (http://www.theglobeandmail.com/news/technology/san-francisco-passes-cell...) came across my desk this week. It talks about a recent vote in by the Board of Supervisors in San Francisco requiring that cell phone retailers post the specific absorption rate (SAR) for all cellular handsets that they sell. The article caught my attention because many cell phone manufactures use our FDTD libraries to model the electromagnetic field strengths, which are used to calculate SAR as a post processing step.

What is SAR?

SAR is a measure of the amount of power that is absorbed by a human body tissue in Watts/kg, averaged over a 1 gram or 10 gram mass of tissue. The FCC and other national government organizations are responsible for defining the safety limits for normal use.

How do you Measure SAR?

Because it is impractical (and inhumane) to insert a probe into someone’s head while they are using a cell phone, we rely on models to determine how much power the human body is absorbing. Since the absorption rate is highly dependent on the position of the cell phone antenna designers will run hundreds, thousands, or even tens of thousands of simulations to determine the SAR value for a given phone.

The other way to measure the SAR value is to create a physical model of the human head and use probes to measure the value. Ideally, simulations match the model results. Major discrepancies need to be resolved before the phone is sent to manufacturing.

Is Lower Better?

Sure. But if the value is too low, your phone will start dropping calls. Typically, antenna designers will work to minimize SAR while maximizing signal strength from the antenna.

During a seismic survey, each shot sends a wave propagating through the Earth, while receivers on the surface listen for reflections as that wave bounces off of geologic layers. Real-world geology can be extremely complex, and because of the different wave propagation velocities the of the different layers, the wave never expands in simple circles, like ripples in a pond. Instead it is scattered off of high-velocity contrasts, refracts around slower regions, is focused into beams.

The purpose of Reverse Time Migration (RTM) is to take those incredibly complex wavefields, as recorded at the surface, and form an image of the underlying geological structure.

The images shown here are Illumination Maps, which show how much energy from a single shot reached each point in the subsurface.

Click to enlarge

There were several information pieces that caught my attention over the last few weeks that seemed to be worthy of sharing. As one of the few non-technical people here at Acceleware, what I appreciated about all of these snipets was how clearly they affirmed the value of the technologies that we are working on. Two of these pieces have a connection with NVIDIA but the third is Intel, so that provides a good balance.

The first one occurred May 18th with IBM and NVIDIA announcing that Big Blue would start incorporating GPU technology in their iDataPlex Servers. It is another great endorsement for using GPUs as part of the processing engines in modern data centers. Check out the video with Scott Denham who gives a very concise overview of the multiple benefits of GPUs.

This past Thursday, April Fools!, I had the honor of speaking at Exxon Mobil’s Technical Software Development conference in Houston. We were one of four external vendors invited to present and later do demos at this all-Exxon event. It was all proprietary so they put a black hood on me, took me to the room after many spins and twists and turns, I gave my presentation, then they hooded me again and escorted me back out of the building.  My talk was right after Dr. Bjarne Stroustrup on C++ and C++0x; definitely a tough act to follow but I did my best.

“One part agile software development, two parts parallel, many-core: Helping Exxon Mobil to reach for higher performance.”

Acceleware's CTO Ryan Schneider introducing our Oil and Gas solutions and it's impact on seismic imaging and reservoir simulations.

Chris Mason, Product Manager at Acceleware, introducing our Linear Algebra products and services:

A major feature was recently incorporated into AxRTM allowing for the propagation of seismic waves in anisotropic media. In seismic jargon, the anisotropy is widely known as tilted transverse isotropy (TTI) whereby the axis of symmetry for wave propagation can have arbitrary tilt which usually corresponds with the dip angle of geological substructures.

In the presence of anisotropy, isotropic migration will incorrectly image the position of seismic structures below dipping anisotropic bedding. TTI is an effective method for correctly imaging the position of geological substructures located below dipping anisotropic overburdens. Imaging with anisotropic modeling capabilities reduces the risk in oil-and-gas exploration drilling decisions. Incorporating anisotropic modeling into vanilla isotropic reverse time migration means that each migration now requires not only the velocity model, but also the Thomsen anisotropy parameters, dip angle and azimuth angle (in the case of 3D migrations).

The BP 2007 TTI model was migrated using both isotropic and TTI anisotropic propagation. Image (a), produced using isotropic AxRTM, shows a lateral shift in the position of the vertical column below the salt. Image (b), produced using TTI anisotropic AxRTM correctly positions the vertical column below the salt. Compare the images with the velocity model shown in image (c).

In conclusion, reverse time migration with TTI anisotropy correctly images the lateral position of vertical columns below dipping anisotropic overburdens. The ability to model with anisotropy provides another tool to confirm the correctness of a given velocity model with more confidence.

Image: (a)

Starting to work in a new team of high-performance computing developers and researchers is like going skydiving. To go skydiving, you first would like to go to skydiving school so that you can at least survive your first skydive. This is where those university years and CUDA courses taught by experienced people become very handy; by no means I am saying that you cannot learn CUDA, skydiving, or anything else on your own but I am saying that with proper training, new abilities can be learned safely and quickly. Once you have all of your training, you go out to jump off as many flying apparatus as you can find; keeping in mind that all you have is training and very little experience. With time, practice, and lots of patience you master your skills; regardless in the air or in front of your computer. The experience that you gather does not make you invulnerable to all the problems that can occur during a skydive or while developing software, but your experience teaches you how you can deal effectively with the many problems that can occur; in skydiving - line over malfunctions, line twists, horseshoe malfunctions, pilot chutes in tow, and in developing high performance software memory leaks, logic errors, race conditions, and problems parallelizing serial algorithms.

Working with the KTM and RTM team at Acceleware has been a great journey over the past few months, just like being at 13000 feet above the ground in a Twin Otter watching as the door slides open and all the noise and wind from the propellers invade the cabin of the plane. You walk to the door and look down to identify the dropzone among all the tiny things on the ground. A sign next to the door catches your eye reading "No Easy Way Down"; and you jump.

Remember to take advantage of all the years of experience that Acceleware has acquired by registering for our training courses, making use of our professional services, and integrating with our products for FDTD, RTM, Medical Imaging, and Matrix.

Thank you, blue skies, and safe programming.

Roberto S. Palacio Jaramillo (53 jumps and counting).

Watch Mike Weldon introducing Acceleware's FDTD solution and it's practical applications:

January 31: So I ended up in Zhuozhou City about 120km from Beijing this week as a guest of the Geophysical Research Institute (GRI). I have trouble following the connection chart, and it changes, but to make a long, convoluted story seem short: it seems like the path is China National Petroleum Company (CNPC), which is then connected to PetroChina, which is then or also connected to the Bureau of Geophysical Processing (BGP), which is then the parent of GRI. GRI is the seismic processing workhorse and I think they do most of the seismic processing for the Chinese oilfields/companies.